The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or may impinge on a retroreflector target that is in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems for determining coordinates of a point are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. A coordinate-measuring device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
Yet there are measurements in which six, rather than just three, degrees of freedom are needed. Here are examples of four such measurements: (1) a six degree-of-freedom (6 DOF) tracker measures the location of a probe tip that is blocked from the view of the tracker by an intermediate object; (2) a 6 DOF tracker follows the motion of a scanning device that measures 3D coordinates using a pattern of light; (3) a 6 DOF tracker finds the orientation, as well as position, of a robot end effector or similar rigid body; and (4) a 6 DOF tracker measures fine object features using a fine probe tip rather than the large spherical surface of an SMR.
Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom. In one system, a camera and laser tracker are used with a target containing a retroreflector and multiple points of light. Exemplary systems are described by U.S. Pat. No. 5,973,788 to Pettersen et al. and U.S. Pat. No. 6,166,809 to Pettersen et al.
In a second system, the target is kept nearly perpendicular to the tracker laser beam by means of motorized or hand adjustment. A beam splitter in the target sends some of the incoming laser light to a position detector, which determines pitch and yaw angles of the target. The rest of the light goes to a retroreflector. Of the reflected light, some passes to a polarizing beam splitter, detectors, and electronics, which determine the target roll angle. The remaining light returns to the tracker. An exemplary system is described by U.S. Pat. No. 7,230,689 to Lau.
A third system is the same as the second system except that the roll sensor is replaced by level sensor that measures the tilt of the target relative to gravity. An exemplary system is described in U.S. Pat. No. 7,230,689 to Lau.
In a fourth system, the tracking device measures the position of a cube-corner retroreflector while also splitting off some of the returning light and sending it to a photosensitive array for analysis. The photosensitive array reads marks intentionally placed on the retroreflector. These marks may, for example, be the intersection lines of the three cube-corner reflection planes. The pitch, yaw, and roll angles of the retroreflector are found by analyzing the pattern displayed on the array. An exemplary system is described in U.S. Pat. No. 5,267,014 to Prenninger.
In a fifth system, an aperture is cut into the vertex of the cube-corner retroreflector. Light passing through the aperture strikes a position detector, thereby providing pitch and yaw angles of the target. The roll is found by one of three means. In the first means, a camera mounted on the tracker measures illuminated points of light in the vicinity of the retroreflector. In the second means, a light source mounted on the tracker emits light over a relatively wide angle, which is picked up by position detector. In the third means, a light source mounted on the tracker projects a laser stripe onto the target. The stripe is picked up by one or more linear arrays. An exemplary embodiment is described in U.S. Pat. No. 7,312,862 to Zumbrunn et al.
Each of these systems of obtaining 6 degrees of freedom (DOF) with a laser tracker has shortcomings. The first system uses a camera to view multiple LEDs in the vicinity of a retroreflector target. A commercial system of this type available today has a camera mounted on top of a tracker. A motorized zenith axis tilts the camera and motorized zoom lens focuses the spots of light. These motorized features are complicated and expensive.
In some implementations of the second system, a two-axis mechanical servo mechanism keeps the target pointing back at the tracker. In other implementations, the user manually points the target toward the tracker. In the first instance, the implementation is complicated and expensive and, in the second instance, the implementation is inconvenient for the user. In addition, the second system uses a polarizing beam splitter, which must be perpendicular to the laser beam for high polarization contrast. For this reason, performance tends to degrade in a handheld system.
In the third system, level sensors respond to tilt (a gravity effect) and acceleration in the same way. Consequently, when a tilt sensor is placed in a hand-held probe, the resulting accelerations caused by hand movement can be mis-interpreted as sensor tilt. To get around this problem, the manufacturers of level sensors sometimes add damping mechanisms (such as damping fluid) to slow the response. Such a damped tilt sensor responds sluggishly to changes in roll angle, which is undesirable.
The fourth system, which reflects light directly from a beam splitter to a photosensitive array to view lines on a cube corner, is limited in its depth of field before the line images on the array become blurry and distorted.
The fifth system requires that an aperture be cut into the retroreflector, thereby degrading retroreflector performance somewhat. It places a position detector, which may be a photosensitive array or a position sensitive detector (PSD), behind the aperture. This aperture is only moderately accurate in the case of the PSD and relatively slow in the case of the photosensitive array. In addition, the system mounts one of three additional means on the tracker. All three means, as described above, are complicated and expensive.
In view of these limitations, there is a need today for a laser-tracker based 6 DOF measuring system that is simple, inexpensive, and accurate.